As a conventional method for producing a conductive film using a conductive paste, there is proposed a method for applying a photosensitive paste, which contains fine inorganic particles, such as fine glass particles, a photosensitive organic constituent, and a compound having an azole structure, such as benzotriazole, on a substrate to expose, develop and fire the applied paste to form a pattern (of a conductive film) (see, e.g., Japanese Patent Laid-Open No. 9-218508). There is also proposed a method for printing a copper ink solution containing copper nanoparticles on the surface of a substrate, and then, causing the printed solution to be dried and exposed to pulses for fusing copper nanoparticles with light sintering, to produce a light-sintered copper nanoparticle film (a conductive film) (see, e.g., Japanese Patent Laid-Open. No. 2010-528428). Moreover, there is proposed a conductive ink using, as a conductive filler, fine copper particles, each of which has benzotriazole deposited on the surface thereof as a process for improving resistance to oxidation (see, e.g., Japanese Patent Laid-Open No. 2008-285761).
However, in the method disclosed in Japanese Patent Laid-Open No. 9-218508, it is required to apply the photosensitive paste on the substrate to cause the applied paste to be exposed and developed with a developing agent, and then, to be fired at a high temperature (520 to 610° C.), so that the process is complicated. In addition, it is not possible to fire the paste with light irradiation, and it is not possible to form the pattern on a substrate, which is easily affected by heat, such as a paper or a polyethylene terephthalate (PET) film. In the method disclosed in Japanese Patent Laid-Open No. 2010-528428, the storage stability of the copper ink solution containing copper nanoparticles is not sufficient, and the adhesion thereof to a substrate, such as a paper, is not sufficient. Moreover, if the conductive ink disclosed in Japanese Patent Laid-Open No. 2008-285761 is used as a conductive paste for light firing, when the solution is applied on the substrate to be dried and fired with light irradiation to form the conductive film, cracks are formed in the conductive film to deteriorate the electrical conductivity of the film.